[PATCH 55/82] kasan: Refactor intentional wrap-around test

[Kees Cook]

In an effort to separate intentional arithmetic wrap-around from
unexpected wrap-around, we need to refactor places that depend on this
kind of math. One of the most common code patterns of this is:

	VAR + value < VAR

Notably, this is considered "undefined behavior" for signed and pointer
types, which the kernel works around by using the -fno-strict-overflow
option in the build[1] (which used to just be -fwrapv). Regardless, we
want to get the kernel source to the position where we can meaningfully
instrument arithmetic wrap-around conditions and catch them when they
are unexpected, regardless of whether they are signed[2], unsigned[3],
or pointer[4] types.

Refactor open-coded wrap-around addition test to use add_would_overflow().
This paves the way to enabling the wrap-around sanitizers in the future.

Link: https://git.kernel.org/linus/68df3755e383e6fecf2354a67b08f92f18536594 [1]
Link: https://github.com/KSPP/linux/issues/26 [2]
Link: https://github.com/KSPP/linux/issues/27 [3]
Link: https://github.com/KSPP/linux/issues/344 [4]
Cc: Andrey Ryabinin <ryabinin.a.a@xxxxxxxxx>
Cc: Alexander Potapenko <glider@xxxxxxxxxx>
Cc: Andrey Konovalov <andreyknvl@xxxxxxxxx>
Cc: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
Cc: Vincenzo Frascino <vincenzo.frascino@xxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: kasan-dev@xxxxxxxxxxxxxxxx
Cc: linux-mm@xxxxxxxxx
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
 mm/kasan/generic.c | 2 +-
 mm/kasan/sw_tags.c | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/mm/kasan/generic.c b/mm/kasan/generic.c
index df6627f62402..f9bc29ae09bd 100644
--- a/mm/kasan/generic.c
+++ b/mm/kasan/generic.c
@@ -171,7 +171,7 @@ static __always_inline bool check_region_inline(const void *addr,
 	if (unlikely(size == 0))
 		return true;
 
-	if (unlikely(addr + size < addr))
+	if (unlikely(add_would_overflow(addr, size)))
 		return !kasan_report(addr, size, write, ret_ip);
 
 	if (unlikely(!addr_has_metadata(addr)))
diff --git a/mm/kasan/sw_tags.c b/mm/kasan/sw_tags.c
index 220b5d4c6876..79a3bbd66c32 100644
--- a/mm/kasan/sw_tags.c
+++ b/mm/kasan/sw_tags.c
@@ -80,7 +80,7 @@ bool kasan_check_range(const void *addr, size_t size, bool write,
 	if (unlikely(size == 0))
 		return true;
 
-	if (unlikely(addr + size < addr))
+	if (unlikely(add_would_overflow(addr, size)))
 		return !kasan_report(addr, size, write, ret_ip);
 
 	tag = get_tag((const void *)addr);
-- 
2.34.1